(1) Field of the Invention
The present invention relates to a display apparatus which usually displays a display object with rectangular red (R), green (G), and blue (B) display elements, each corresponding to one pixel. More particularly, the present invention relates to a display apparatus, a display method, a display controller, and a letter image generation device for use in displaying high resolution letters (i.e., small letters), as well as to a computer-readable recording medium in which a letter image generation program is performed.
(2) Description of the Related Art
In association with recent pursuit of a lighter-weight display apparatus (personal computer) flat-panel type typified by a liquid crystal color display apparatus, use of the display apparatus in a transportable manner has now become predominant. Against such a backdrop, there has been sought display of high-resolution letters and a color image display on a smaller screen.
For instance, indication of annotations or Japanese kana characters is indispensable for displaying Japanese contents, such as contents of books and magazines. Japanese kana characters are displayed in substantially half or less the size in which a text is to be displayed (e.g., in 6-point or smaller letters when text is being displayed in 12-point letters).
Indication of such Japanese kana letters on a display apparatus requires a resolution of 180 dpi (dots per inch) or more. Even when a conventional color display of flat-panel type typified by a liquid crystal display is used in a portable terminal, it is difficult to achieve such a high resolution.
A known related-art method for displaying such high-resolution monochrome letters is a halftoning technique using grayscale fonts or sub-pixel fonts.
In connection with grayscale fonts, edges of a letter are displayed in multiple shades of gray, and a font is produced by utilization of halftones. Jaggies are suppressed by reducing inconsistencies in density in the edges and smoothing the edges with upper, lower, left, and right adjacent patterns. Even when pixels are relatively large compared with the display size of letters, the letters can be read. In other words, jagged edges of a letter (i.e., jaggies) can be lessened.
FIGS. 21A through 21C are illustrations for describing a method of forming grayscale fonts (i.e., a halftoning technique). FIG. 21A is an enlarged view showing a portion of a letter image before processing. FIG. 21B is a view showing an example of a smoothing filter to be used for forming a grayscale font. FIG. 21C is an enlarged view showing a portion of a formed grayscale font.
According to the halftoning technique to be used for forming a grayscale font, smoothing filters formed from a 3×3 pixel matrix shown in FIG. 21B (wherein 1/16 ⅛ 1/16, ⅛ ¼ ⅛, 1/16 ⅛ 1/16) are superimposed on a letter image formed from two shades of gray shown in FIG. 21A. As a result, halftone fonts (grayscale fonts) such as those shown in FIG. 21C are formed.
Provided that a letter image before being halftoned is denoted as F, a smoothing filter is denoted as “f,” a grayscale font to be formed is denoted as Fg, and a superimposing operation is denoted as , the method of forming grayscale fonts can be expressed as follows:Fg=Ff
where, f=( 1/16 ⅛ 1/16, ⅛ ¼ ⅛, 1/16 ⅛ 1/16)
In connection with sub-pixel fonts, letters are halftoned by individual use of R-G-B elements and by dispersing the value of each pixel in a horizontal direction.
Provided that a letter image before being halftoned is denoted as F, an energy dispersion coefficient is denoted as ε (e.g., ε=0.11, 0.22, 0.33, 0.22, 0.11), a sub-pixel font to be produced is denoted as Fs, and a superimposing operation is denoted as , the method of forming a sub-pixel font can be expressed as follows:Fs=Fε
In connection with sub-pixel fonts;, the size into which a letter is half toned by use of the energy dispersion coefficient ε is determined on a per-element basis. Hence, halftoning of a letter using a sub-pixel font yields improved resolution as compared with halftoning of a letter using a grayscale font.
When a display apparatus displays Japanese document contents, text of the contents is displayed in a point size of, e.g., 10 (10 dots at 72 dpi). In this case, Japanese kana characters above kanjis must be displayed with letter images of about half the size of the letters.
However, in the related-art halftoning method, it is difficult to resolve a 5-point letter. For instance, when letters of point size 5 or thereabouts are displayed on a common liquid crystal display apparatus, letter images are displayed in the form of an about 6×6 pixel matrix or a 7×7 pixel matrix in the resolution of about 100 pixels/inch (dpi).
In this case of such halftoning method, an interval (a stroke pitch) at which pixels constituting a letter come closest to each other corresponds to one pixel. When letters of point sizes 5 or less are displayed on a display apparatus having a resolution of 100 dpi or thereabouts, pixels constituting a letter are merged, thus posing difficulty in letter recognition.
The halftoning technique using grayscale fonts involves spread of a minimum of three pixels (i.e., collapse of a stroke) arising in both horizontal and vertical directions. Even in the case of a halftoning method using sub-pixel elements, spread of five elements (i.e., 5/3 pixels) arises in a horizontal direction (i.e., a direction in which R-G-B elements are to be arranged). As a result, pixels constituting letters are merged, thereby posing difficulty in letter recognition.